1. Field of the Invention
The present invention relates to an insulated gate bipolar transistor having high current density and a reduced on-voltage.
2. Description of the Related Art
Low power consumption power conversion apparatuses have recently been promoted. Thus, research into low power consumption by a power semiconductor device that performs a central role in the power conversion apparatus has been actively undertaken.
In particular, among power semiconductor devices, research into an insulated gate bipolar transistor (IGBT) is actively ongoing, since an IGBT can achieve low on-voltage according to a conductivity modulation effect and can be easily controlled owing to a voltage-driven gate.
Types of IGBT include a planar IGBT, a trench IGBT, and others. A planar IGBT has a structure in which gate electrodes are formed on a wafer surface. A trench IGBT has a structure in which an oxide film is interposed in trenches formed vertically downward from a wafer surface and gate electrodes are buried therein.
A trench IGBT includes channels formed in both inner walls of the trenches, which may increase channel density as compared to the case of a planar IGBT. Thus, the trench IGBT can further reduce an on-voltage.
The structure of a conventional trench IGBT may include an n− type low concentration silicon substrate that is a drift layer, an n type field stop layer formed in one surface of the n− type drift layer, and a relatively high concentration p type thin collector layer formed in one surface of the field stop layer in which an amount of impurities is controlled.
A plurality of p type base areas are formed in the other surface of the n− type drift layer. Surfaces of the p type base areas have n+ type emitter areas selectively formed thereon.
The trench is formed from the n+ type emitter area to the n− type drift layer through the p type base area. The gate electrode formed of conductive polycrystalline silicon is formed inside the trench having a gate oxide layer therebetween.
An interlayer insulation layer coated on an upper portion of the trench insulates an emitter electrode and the gate electrode.
The emitter electrode formed on an upper portion of the interlayer insulation layer is formed commonly conducting and contacting the n+ type emitter area and the p type base area by using an aperture window installed in the interlayer insulation layer.
A collector electrode is installed in a rear surface of the p type collector layer.
To allow the trench IGBT to be on-state, a voltage higher than a threshold voltage needs to be applied to the gate electrode in a state in which a voltage applied to the collector electrode is higher than a voltage applied to the emitter electrode.
Charges are accumulated in the gate electrodes by the above-described voltage and an n type inverted channel is concurrently formed in a surface of the p type base area side facing the gate oxide layer interposed between the gate electrodes.
Electrons are injected into the n− type drift layer from the n+ type emitter area through the n channel. The injected electrons make collector bonding forward biased, holes are injected from the p type collector layer, and thus the trench IGBT becomes the on-state.
A voltage drop value between the collector electrode and the emitter electrode in the on-state is equal to an on-voltage.
To change the IGBT from the on-state to an off-state, the voltage of the gate electrode needs to be below a threshold value.
Through the change in states, the charges accumulated in the gate electrodes are discharged to a gate driving circuit through a gate resistor. In this regard, since the n type inverted channel area is converted into a p type area, a path of the electrons disappears, resulting in no electron supply to the n− type drift layer. Accordingly, hole injection from the collector layer is not made, and thus, the electrons and the holes accumulated in the n− type drift layer are discharged to the collector electrode and the emitter electrode, respectively, or are re-coupled to each other. Thus, current is extinguished, and the IGBT becomes the off-state.
A variety of attempts have been made to increase a current density of the trench IGBT and further reduce the on-voltage. Conventionally, performance enhancement of the IGBT has been promoted by mainly adjusting a trench space of the IGBT.
However, narrowing of the trench space is restricted due to limited photo processing. Thus, a new method, other than the method of narrowing the trench space needs to be proposed.